Bangkok: An Australian forensic scientist has questioned key DNA evidence that led to two migrant workers being sentenced to death for the gruesome murders of two British backpackers on an idyllic Thai island.

Myanmar workers Win Zaw Htun, right, and Zaw Lin, left, both 22, escorted by officials after they were convicted in December last year of the murder Hannah Witheridge and David Miller. Photo: AP

Melbourne-based Jane Taupin says documents detailing how Thai investigators matched DNA from Myanmar workers Zaw Lin and Win Zaw Htun to the victims were not provided to a Thai court, in contravention of international DNA analysis and reporting standards.

She also points out that DNA matching, a complex procedure requiring meticulous care, can only be determined on the basis of statistical probability in the population and none was presented to the court.

Zaw Lin, left, and Win Zaw Htun during an October 2014 police press conference after their arrest on Koh Tao, Thailand. Photo: AP

Police said the pair confessed to the killings but both men later retracted their statements, saying they were tortured.

After a 21 day trial, three judges of the Samui Criminal Court found that that the accused 22-year-olds bludgeoned Mr Miller to death with a hoe and then jointly raped Ms Witheridge while she lay unconscious after a late night beach party.

Protesting in Yangon, Myanmar, in December last year against a Thai court's verdict sentencing two Myanmar migrant workers to death. Photo: AP

In a review of the DNA evidence obtained by Fairfax Media, Ms Taupin said no documents detailing the collection, movement, handling and chain of custody of DNA samples were provided to the court which is required under United States and United Kingdom codes of practice.

"The scientific records were not provided for review (for whatever reason) and thus as a scientist I could not perform a scientific review, or determine whether these records accorded with the principles of the standard," she said.

Myanmar citizens rallying against a Thai court's verdict sentencing two Myanmar migrant workers to death for the murder of British backpackers Hannah Witheridge and David Miller. Photo: AP

Ms Taupin, an independent consultant who has examined DNA evidence for police agencies in Australia and the UK and has received several forensic science awards, travelled to Thailand expecting to testify in the case in July but she was not called to the stand.

Thailand's best-known forensic expert Pornthip Rojanasunand testified for the defence that the crime scene had been poorly managed and the collection of evidence "contradicted the principles of forensic science".

The prosecution provided only a one-page summary of their DNA tests, some of it handwritten, with parts crossed out and corrected, along with four supporting pages.

Ms Taupin said case file notes from the Thai police forensic laboratory should have been produced that showed a continuity of exhibits and the rationale for any scientific testing.

"Without these, any scientific review is limited and thus itself does not achieve a proper standard," she said.

Ms Taupin stressed her comments were not criticism of the court but a review of the DNA evidence that defence lawyers say will form the basis of an appeal.

Andy Hall, an advocate for migrants who advised defence lawyers in the case, said the defence requested additional DNA-related documents from the prosecution but they were not provided.

"I can confirm there were no documents in the case file that explained methods of DNA testing or any assumptions from the police side or explanation of results in detail," Mr Hall said.

"There were also no probability statistics provided for matches of DNA and nor was there any documents at court specifying evidence to support the substance/biological nature of the DNA profile," he said.

Mr Hall said the prosecution referred to DNA coming from sperm, saliva and skin but provided no evidence to back up the assertions.

He said the defence lawyers are confused how concerns about the DNA evidence were not raised or noted by the UK government which sent a team of detectives to review the investigation.

David Miller's brother Michael told reporters minutes after the judges delivered their verdict that justice had been delivered.

Thailand's military ruler Prayuth Chan-ocha has also angrily dismissed criticism of the case.

"They have a right to appeal, don't they? Isn't this the same legal practice all over the world?," he said.